FIG. 15 is a perspective view of a prior art thin-film solar cell in which a light-to-electricity conversion is carried out in a thin silicon film disposed on a substrate. In FIG. 15, a thin-film active layer 101 including a p-n junction and contributing to power generation is disposed on a substrate 100. An anti-reflection film 102 is disposed on the active layer 101. A grid electrode 103a for collecting a photoelectric current generated in the active layer 101 and a bus electrode 103b for concentrating the photoelectric current are disposed on the anti-reflection film 102. A lower electrode 104 is disposed on the rear surface of the substrate 100.
In this thin-film solar cell, since the active layer 101 contributing to power generation is as thin as several tens of microns, it cannot mechanically support itself, so that a substrate or the like for supporting the thin active layer 101 is needed. The following conditions are required of the substrate.
First, the substrate should have a strength enough to mechanically support the thin film and itself. Second, since the Si thin-film active layer is grown on the substrate by thermal CVD or the like, the substrate should be refractory so that it can stand a process temperature of approximately 1000.degree. C. during the growth of the active layer. Third, since the substrate also serves as a lower electrode, it should be electrically conductive. Even if the substrate is not conductive, a thin-film solar cell can be achieved. In this case, however, a conductive film must be disposed on the substrate or the lower electrode must be led out from the side surface of the solar cell in an integrated type solar cell, resulting in a complicated structure. Fourth, since the substrate itself does not contribute to power generation but only supports the active layer, it is desirable that the substrate is formed in a simple process using an inexpensive material.
As a material satisfying the above-described conditions, there is metallurgical grade silicon (hereinafter referred to as MG-Si). The MG-Si is a silicon material prior to being purified to make high purity silicon and includes a lot of impurities, i.e., an impurity concentration of about 2%. Since the MG-Si is not subjected to purification, it is much cheaper than high purity silicon.
FIGS. 16(a) and 16(b) are schematic diagrams illustrating a method of producing a heat-resistant supporting substrate of a thin-film solar cell using the inexpensive MG-Si. As shown in FIG. 16(a), MG-Si powder 50 is put in a mold 110, and the mold is heated to a temperature higher than the melting point of silicon, i.e., 1414.degree. C., to melt the MG-Si powder. Then, the MG-Si thus melted is pressed with a plate 111 as shown in FIG. 16(b) and, thereafter, it is cooled and solidified to manufacture a MG-Si substrate 5.
FIG. 17 is a cross-sectional view of a prior art thin-film solar cell including the MG-Si substrate 5 formed by molding. In FIG. 17, a polycrystalline Si thin-film active layer 2 is disposed on the MG-Si substrate 5. A p-n junction 3 is produced in the surface region of the active layer 2 by diffusion or the like. An upper electrode 4 is disposed on the active layer 2 having the p-n junction 3.
A description is now given of a method of producing the thin-film solar cell shown in FIG. 17 using the MG-Si substrate 5. Initially, the MG-Si substrate 5 is put in a CVD apparatus. Then, silane (SiH.sub.4) gas, silane trichloride (SiHCl.sub.3) gas, or the like is introduced into the apparatus and decomposed at a high temperature of about 1000.degree. C., whereby a polycrystalline Si film to be the active layer 2 is grown on the substrate 5 to a thickness of several tens of microns. Since the Si thin film Just after the growth has a small grain size, the polycrystalline Si may be, in some cases, melted and recrystallized by laser radiation or radiant heating to increase the grain size. After forming the Si film, the p-n junction 3 is produced in the active layer 2 by dopant impurity diffusion or ion implantation. The p-n junction may also be produced by changing the kind of dopant gas while introducing a dopant gas into a CVD apparatus to and growing the active layer in the apparatus. Alternatively, the p-n junction may be produced by depositing on the active layer a microcrystalline film having a conductivity type opposite the conductivity type of the active layer in a plasma CVD apparatus.
After forming the p-n junction, the upper electrode 4 comprising silver or the like is formed on the active layer 2. Preferably, the upper electrode 4 is formed by screen printing or vapor deposition. There are some cases where an anti-reflection film is formed on the polycrystalline Si thin film by sputtering or the like. As the anti-reflection film, a transparent conductive film also serving as an electrode, such as an ITO (In.sub.2 O.sub.3 :SnO.sub.2) film, a SnO.sub.2 film, or a ZnO film, is used when the p-n junction is produced by depositing a microcrystalline film on the Si film and the conductivity of the Si film in the transverse direction is low. When the conductivity of the Si film in the transverse direction is high and the transparent electrode is not needed, an insulating film, such as a Si.sub.3 N.sub.4 film, is used as the anti-reflection film.
In the above-described method of producing the thin-film solar cell using the molded MG-Si substrate 5, the MG-Si substrate is heated up to about 1000.degree. C. when the polycrystalline Si thin film is grown and, thereafter, it is heated up to about 1414.degree. C. when the grain size of the Si thin film is increased by radiant heating or the like. Therefore, impurities, such as Fe, Al, Ca and the like, included in approximately 2% in the MG-Si substrate 5 unfavorably concentrate and sprout out of the substrate thereby breaking through the active layer.
This phenomenon is illustrated in FIG. 18. In FIG. 18, reference numeral 5 designates the molded MG-Si substrate, numeral 2 designates the polycrystalline Si thin-film active layer, numeral 3 designates the p-n junction, and numeral 6 designates the sprouting impurities. As shown in FIG. 18, when heat is applied to the molded substrate 5, the impurities concentrate and sprout from a portion supposed to be a grain boundary of the MG-Si substrate, breaking through the active layer 2.
In addition, the p-n junction of the active layer is formed by impurity diffusion or ion implantation, or by changing the kind of the dopant gas during the growth of the active layer in a CVD apparatus, or by depositing a microcrystalline film having an opposite conductivity type from the conductivity type of the active layer on the active layer in a plasma CVD method. In all cases, the formation of the p-n junction takes a lot of time.